Lion's Mane jellyfish showing up in "unusually large numbers" off Cape Cod, harbormaster says

Giant jellyfish are becoming a popular sight at Massachusetts beaches this summer.

The Sandwich harbormaster said Tuesday that Lion's Mane jellyfish, the largest jellyfish species in the world, "are currently appearing in unusually large numbers off Cape Cod."

"While Lion's Mane is normally seen from late spring through summer, this year reports began in May, with exceptionally high densities observed by mid-June," the harbormaster posted to Facebook. "Experts believe factors such as water temperatures and available food sources may contribute to these increases."

The last time the jellyfish were seen in such large numbers in Massachusetts was in 2020. 

What are Lion's Mane jellyfish?

Lion's Mane jellyfish are reddish-brown and can grow to more than 7 feet across, and their tentacles can stretch as long as 100 feet, though they can be difficult to see underwater.

The New England Aquarium also said Wednesday that they've been spotted on Massachusetts beaches and in local waters. Late last month, The BCYF Curley Community Center in South Boston restricted beach access after some swimmers got stung. 

"Keep an eye out: lion's mane jellies can give a pretty painful sting!" the aquarium said. "If you see one washed up on the beach or floating in the water, it's best to admire it from a safe distance."

What to do if stung by a jellyfish

Lion's Mane jellyfish can sting people even after they've washed up on shore. While the stings are painful, the harbormaster said they are not particularly dangerous. 

"If stung, rinse with warm water and carefully remove tentacles with tweezers," the harbormaster said.

Any sightings of Lion's Mane jellyfish at beaches should be reported to lifeguards.